A floating gate (FG) type structure is typically employed in a cell structure of a NAND-type flash memory, for example, which is part of a nonvolatile semiconductor memory device. In the FG type structure, the operation window of a memory device is wide and its ability to store charge is excellent.
However, in recent years, cells are being shrunk and it is hard to adjust the physical and/or electrical properties of a memory cell transistor and the physical and/or electrical properties of a transistor of a peripheral circuit at the same time.